Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically-actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.
CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.
Referring initially to FIG. 1, a conventional CMP apparatus 50 includes a conditioning head 52, a polishing pad 56, and a slurry delivery arm 54 positioned over the polishing pad 56. The conditioning head 52 is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 56 for making a sweeping motion across the entire surface of the polishing pad 56. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad 56 during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after prolonged use.
The slurry solution is typically distributed to the slurry dispensing nozzles 62 through tubing (not illustrated), by operation of a pump (not illustrated). Currently, no system exists for accurate in-situ monitoring of the flow rate and precise control of the flow rate of the slurry solution from the pump to the CMP apparatus. Excessively high flow rates of the slurry to the CMP apparatus tend to waste the slurry, whereas excessively low flow rates of the slurry to the CMP apparatus causes inadequate supply of the slurry to the wafer, and thus, hinders optimum polishing. Additionally, slurry flow rates which are characteristic of conventional systems are frequently variable and unstable. Moreover, conventional slurry delivery systems are typically incapable of controlling the mixing ratio of the slurry components. Accordingly, a system is needed for accurately monitoring the flow rate of slurry from a slurry pump to a CMP apparatus and precisely controlling the flow rate of the slurry to the apparatus for economical and optimum chemical mechanical polishing.
An object of the present invention is to provide a system for monitoring the rate of delivery of a fluid to a destination.
Another object of the present invention is to provide a system for controlling the rate of delivery of a fluid to a destination.
Still another object of the present invention is to provide a system for both accurately monitoring and precisely controlling the rate of delivery of a fluid to a destination.
Another object of the present invention is to provide a system which facilitates control in mixing slurry components.
Yet another object of the present invention is to provide a system for delivering a fluid to a destination in a substantially bubble-free condition.
A still further object of the present invention is to provide a system for removing gas bubbles from a liquid and monitoring and controlling the rate of delivery of the liquid to a destination.
Still another object of the present invention is to provide a closed-loop system for in-situ monitoring and controlling of the rate of delivery of a polishing slurry to a chemical mechanical polisher.
Another object of the present invention is to provide a system for improving the dishing range for chemical mechanical polishers.
Yet another object of the present invention is to provide a system which is capable of reducing the rate of flow of polishing slurry to a chemical mechanical polisher to avoid or reduce wasting of the slurry.
A further object of the present invention is to provide a novel degasser for removing gas bubbles from a polishing slurry or other liquid.